Overvoltage protection materials and process for preparing same

ABSTRACT

The invention provides a process for preparing an overvoltage protection material comprising: (i) preparing a mixture comprising a polymer binder precursor and a conductive material; and (ii) heating the mixture to cause reaction of the polymer binder precursor and generate a polymer matrix having conductive material dispersed therein, wherein the polymer binder precursor is chosen such that substantially no solvent is generated during the reaction.

FIELD OF THE INVENTION

The present invention relates to materials for protecting sensitiveelectronic circuits against transient high electrostatic pulses. Moreparticularly, it relates to a new solvent-free methodology for producingthese materials, and devices comprises these materials.

BACKGROUND OF THE INVENTION

Transient high voltages can induce harmful currents and voltages inelectronic circuits and electric equipment. Normally, these transientvoltages are caused by electrostatic discharge, lightning or inductivepower surges.

Overvoltage protection materials and devices have been developed whichaim to protect such electronic and electrical equipment from thesetransient high voltages. The overvoltage materials have non-linearelectrical resistance characteristics, specifically having a very highelectrical resistance (e.g. >30 MOhm) at a normal operating voltages(e.g. <200 V for normal electronic instrument), but switching to anessentially conducting state when subject to a transient high voltage.The voltage at which the material changes from its high resistance state(or “off state”) to its conducting state (or “on state”) is known as thethreshold value, or trigger voltage. It is necessary for the materialsto have a very fast switching time between the high resistance state andconducting state in order to provide adequate protection for theelectronic or electrical equipment being protected. An acceptableswitching time is usually of the order of nanoseconds, and is preferablyless than one nanosecond. The device returns back to its normal highresistance state after the transient high voltage threat has passed.

The term overvoltage protection material usually refers to a compositematerial containing a conductive material within a polymer matrixsystem. The overvoltage protection material may comprise only a twocomponents (the conductive material and the polymer matrix), or cancontain other components such as a semi-conductive material or anon-conductive material.

Conventional methods for preparing overvoltage protection materialsinclude solvent processing or polymer condensation processes. Usually,the finished polymeric systems should be a network structure or athermoset in order to achieve good thermal properties. Therefore, bothof these processing techniques should be carried out in the presence ofa cross-linking component.

(a) Solvent Processing Method

One method of making thermoset plastics suitable for use as overvoltageprotection materials is a solvent processing method, as described inU.S. Pat. Nos. 4,977,357 and 5,248,517. The overvoltage conductivematerials are prepared by mixing a dissolved silicone polymer solution,nickel powder, silicon carbide powder; inorganic filler andcross-linking agent (e.g. organic peroxide) to form a paste likecomposition. This material is then applied to a prepared substrate whichcontains metal electrodes, to form an uncured device. The device is thencured in an oven at 125° C. for 4 hours.

However, this solvent processing technique has a number ofdisadvantages. First, the polymer must be dissolved in a suitablesolvent, the choice of which is critical to the success of the reaction.Not all solvents can be used (e.g. all chlorinate and some ketone-typesolvents are unsuitable), and it can take a great deal of time todetermine which solvents are suitable. Second, a relatively large amountof solvent must be removed (e.g. sometime as much as 1:1, solid tosolvent weight content) under the high temperature curing step. Removalof solvent can be damaging to the device, generating voids withinpolymer matrix and causing a resulting degradation in performance of thefinal overvoltage protection device. Third, the processing temperaturecondition required to evaporate the solvent is relatively high, and thecuring time is relatively long (e.g. approximately 4 hours at 125° C.).

(b) Polycondensation Process Method

U.S. Pat. No. 5,928,567 discloses a liquid conductive material designedto protect electrical components from high pulse static electricity. Theprocess comprises combining a solvent-free liquid silicone polymercomposition (General Electric RTV11), a conductive metallic powder, anon-conductive inorganic powder and a catalytic amount of the curingagent (e.g. dibutyl tin dilaurate) and mixing these components in aconventional multi-blade mixer. The paste like material is applied to anappropriate substrate, and is then cured in a convection oven at 80° C.for 2 hours. The process is described as solvent free because noconventional organic solvent is added in the process described.

However, while no solvent is added to the mixture, the process itselfwill generate solvent as a by-product. In the process specificallydescribed in this patent, the polymerization process is a commoncondensation reaction between di- or tri-hydroxy polydimethylsiloxaneand di- or tri-methoxy polydimethylsiloxane prepolymers. A generalreaction scheme is shown below in Scheme 1.

From this it can be seen that, although a solvent is not required as aninitial component of the reaction mixture, some solvent (in this casemethanol) will be generated during the course of the curing reaction. Inthis instance, one mole of solvent is generated for every mole ofproduct. During the curing reaction the solvent present in the mixturewill evaporate and in doing so will generate unwanted voids or bubblesin the polymer matrix. This in turn diminishes the ability of the deviceto respond properly during conditions of transient high voltage.

From a consideration of the prior art processes it can be seen thatperformance of overvoltage protection devices is reduced due to thepresence of solvent either as an added component of the reactionmixture, or as a by-product formed during the curing process. It istherefore an object of the invention to provide an improved andconvenient method for preparing overvoltage protection materials. Inparticular, it is an object of the invention to provide a solvent-free,or “clean”, process for preparing an overvoltage protection materialsuitable for protecting electronic circuits and electrical devices fromtransient high voltages, where no solvent is added to the reactionmixture, and where no substantially solvent is generated during thecourse of the process. It is another object of the invention to providean improved overvoltage protection material. It is also an object of theinvention to provide new overvoltage protection devices comprising thisimproved overvoltage protection material.

SUMMARY OF THE INVENTION

The invention provides a process for preparing an overvoltage protectionmaterial comprising:

-   -   (i) preparing a mixture comprising a polymer binder precursor        and a conductive material; and    -   (ii) heating the mixture to cause reaction of the polymer binder        precursor and generate a polymer matrix having conductive        material dispersed therein,        wherein the polymer binder precursor is chosen such that        substantially no solvent is generated during the reaction.

The invention also provides a process for preparing an overvoltageprotection device comprising:

-   -   (i) depositing a metallic layer on a substrate, and etching the        metallic layer in order to provide metallic electrodes separated        by gaps;    -   (ii) preparing a mixture comprising at least one polymer binder        precursor and a conductive material, and applying this mixture        between and in contact with adjacent electrodes; and    -   (iii) heating the device to cause reaction of the polymer binder        precursor and generate an overvoltage protection material        comprising a polymer binder having conductive material dispersed        therein;        wherein the polymer binder precursor is chosen such that        substantially no solvent is generated during the reaction.

In another embodiment, the invention provides an overvoltage protectionmaterial preparable according to a process comprising:

-   -   (i) preparing a mixture comprising at least one polymer binder        precursor and a conductive material; and    -   (ii) heating the mixture to cause reaction of the polymer binder        precursor and generate a polymer matrix having conductive        material dispersed therein,        wherein the polymer binder precursor is chosen such that        substantially no solvent is generated during the reaction.

In a further embodiment, the invention provides an overvoltageprotection material comprising a polymer binder having a conductivematerial dispersed therein, said overvoltage protection material beingobtainable by an addition polymerisation reaction of a mixturecomprising a polymer binder precursor and a conductive material.

In each embodiment above, the mixture of polymer binder precursor andconductive material is preferably substantially solvent-free. Theinvention also relates to the polymer binders described and prepared inthe methods above.

The invention also provides an overvoltage protection materialcomprising a polymer binder having a conductive material dispersedtherein, said material being substantially free of voids.

Finally, in another embodiment the invention provides a circuitcomprising electronic components, at least a first and second conductingregion, an amount of an overvoltage protection material being disposedbetween and in contact with said conducting regions, the overvoltageprotection material being as described above.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross-sectional view of an overvoltage protection device 10.

FIG. 2 is a plan view of an overvoltage protection device substratewithout an overvoltage protection material.

FIG. 3 is a plan view of an overvoltage protection device containing anovervoltage protection material 15.

FIG. 4 is a diagram of a testing circuit for measuring the electricalproperties of the overvoltage protection materials according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention describes new processes for producing overvoltageprotection materials, and more specifically describes some very uniqueand clean organic synthetic methodologies for preparing cross-linkedpolymeric and highly robust networked systems, and to the formation ofconductive composite matrix materials which are free of unwantedcontaminants. The materials produced according to the invention areuseful as overvoltage protection materials, behaving like electricalinsulators under normal operation, but switching to became conductorsunder high transient voltage. The materials also have a high degree ofheat resistance (many thermoplastic polymers become soft or even melt attemperatures of about 80° C. to about 200° C.). Furthermore, thesimplified processing steps used in the process of the invention reduceunnecessary contaminants in the final cured composite materials.

The process of the invention relates to the synthesis of a highlycross-linked polymer system which contains a conductive material, withthe resulting composite material being useful as an overvoltageprotection material. During the process, no solvent is required in orderto produce the resulting composite material, nor is any substantialamount of solvent generated as a by-product of the reaction. The phrase“substantially no solvent” is intended to mean that no significantamount of solvent is generated during the reaction. If any solvent isgenerated, this is present in a very small amount, generally less thanabout 2 wt %, and preferably less than about 1 wt %, based on the totalweight of the composite material. Obviously it is preferred to preventany solvent being generated at all.

Reducing the amount of solvent generated during the process has a numberof advantages. For example it reduces the likelihood of unwanted voidsor bubbles being produced in the resulting composite material, suchvoids or bubbles being caused by fast evaporation of solvent during thecuring process. By reducing the presence of these irregularities in thecomposite material, the electrical properties of overvoltage protectiondevices made from this material are improved. When an overvoltageprotection material of the invention is described as being substantiallyfree of voids, this is intended to mean that voids account for less than5% of the volume of the material, preferably less than 1% of the volumeof the material. Clearly, it is advantageous to ensure that the materialis entirely free of voids, however if voids are present, they arepreferably less than about 1 mm in diameter, more preferably less thanabout 1 μm in diameter.

Polymeric Binder

The polymeric binder used in the invention serves two major roles: firstit acts as a media for separation of the conductive particles, andsecond it provides enhanced thermal resistance against shape deformationof the polymer matrix system. The term “polymer binder” when used hereinrefers to a cross-linked polymer structure, also known as a polymermatrix, in which is dispersed the conductive material. The polymerbinder is prepared from a polymer binder precursor. The term “polymerbinder precursor” represents a precursor composition which, uponheating, reacts to form the cross-linked polymer binder, or polymermatrix. The precursor may comprise a single type of monomer, oligomer orprepolymer, or may comprise a mixture of different monomers, oligomersand/or prepolymers. It may also comprise other components which may berequired to form the polymer binder, such as cross-linking agents and/orcatalysts.

There are numerous techniques and synthetic methodologies known in theart for producing thermoset polymer systems. However, it is arequirement of the present invention that the reaction takes placewithout a significant amount of solvent being generated during thereaction. Furthermore, it is preferred that the entire reaction iscarried out under substantially solvent-free conditions (i.e. withsubstantially no solvent being present at the beginning of the reaction,and substantially no solvent being generated during the reaction). Asynthetic route which is therefore suitable is an additionpolymerization reaction. Such a reaction may be carried out in theabsence of solvent (i.e. no solvent is required to dissolve thereactants prior to curing).

Addition polymerization reactions differ from those described in theprior art such as U.S. Pat. No. 5,928,567 because substantially nosolvent is generated as a by-product. As shown in Scheme 1 above,polycondensation reactions involve the reaction of two molecules withthe elimination of a solvent molecule, thus a significant amount ofsolvent is generated as a by-product. In contrast, the polymer additionreactions of the present invention involve the reaction of at least twomolecules without elimination of a solvent molecule.

The polymer binders of the present invention can be prepared from avariety of different polymerizable functional monomers, oligomers orprepolymers according to a number of different methodologies. When usedin the present invention, the polymer binder precursors are reacted toform the polymer binder in the presence of the conductive material, inorder to form a matrix having conductive material dispersed therein.However, the following addition polymerisation reactions will bedescribed generally, without stating the presence of the conductivematerial which will, in the processes of the invention, always bepresent.

Reactions which are particularly suitable in the process of theinvention include bulk free radical polymerisation, polyurethanesynthesis, epoxy resin synthesis and polyhydrosilylation.

Bulk Free-Radical Polymerization:

Many “bulk” (i.e. solvent-free) free-radical polymerisation reactionsare known and could be used in the present invention. Some reactionswhich are particularly useful in the present invention involve alkylacrylates or alkyl methacrylates monomers or liquid formed prepolymerscarrying acrylate or methacrylate functional groups.

In a typical “bulk” free radical polymerization, monomers or oligomersof acrylates or methacrylates may be reacted in the presence ofmethacrylate- or acrylate-based cross-linking agents and a free radicaltype catalyst (e.g. peroxides or AIBN). The formation of the copolymercan be best represent by the following reaction scheme 2:

The acrylate and methacrylate components are preferably selected fromthe group consisting of methyl acrylate, ethyl acrylate, butyl acrylate,2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, butylmethacrylate and 2-ethylhexyl methacrylate.

The crosslinking agent is a compound having more than one functionalgroup, and is preferably selected from the group consisting of1,4-butanediol diacrylate, 1,4-butylene glycol diacrylate, ethylenediacrylate, 1,6-hexamethylene diacrylate, 1,4-butanediol dimethacrylate,1,4-butylene glycol dimethacrylate, ethylene dimethacrylate,1,6-hexamethylene dimethacrylate, trimethylpropane trimethacrylate,pentacrythediol tetraacrylate and pentacrythediol tetramethacrylate.

In general, the reaction can be easily initiated by heating the contentmixture. Normally, the initiating temperatures are all depends on thetype of the radical initiator use which can be ranging from 40° C. toover 150° C.

Polyurethanes Synthesis:

Polyurethanes are among the most common polymers in the global polymermarket. Polyurethanes can be synthesised according to a well establishedaddition process between isocyanates and polyols. A wide range ofisocyanate and polyol compounds are known in the art, and thesecompounds may contain aliphatic and/or aromatic moieties.

Polyurethane formation usually occurs via a step growth polymerizationprocess in which the chain length of the polymer increases as thereaction progresses. The polymer may be a linear, or slightly branched,thermoplastic material, or may be in the form of a cross-linkedthermoset network. To be suitable for use in the overvoltage protectiondevices of the present invention, the polyurethane should be anelectrical insulator.

The most widely used method of synthesising polyurethanes is by thereaction of a diisocyanate and a polyol, and this synthetic reaction maybe used in the present invention. Thus, in a preferred embodiment thepolymer binder precursor comprises a diisocyanate component and a polyolcomponent. These components can react together to form a polyurethanepolymer binder. The reactants are preferably used in their liquid forms,and may be present as monomers, oligomers or prepolymers.

The diisocyanate component is preferably selected from the groupconsisting of 1,4-diisocyanatobutane, 1,6-diisocyanatohexane,1,8-diisocyanatooctane, 1,12-diisocyanatododecane, 2,4-toluenediisocyanate, isophorone diisocyanate terminated poly(1,4-butandiol),tolylene 2,4-diisocyanate terminated poly(1,4-butandiol), and othercommercially available diisocyanates such as those available under thetrade marks Desmodur® L67BA, ISONATE® M342 and ISONATE® 143L. Thediisocyanate component is preferably present in the reaction mixture inan amount of from 5 to 50 wt %, based on the total weight of thereactants.

The polyol component may be a diol, and is preferably selected from thegroup consisting of short chain dihydroxy alcohols such as1,2-ethandiol, 1,3-propandiol, 1,2-propandiol, 1,4-butandiol. The polyolcomponent may also be selected from the group consisting ofpoly(ethylene glycol) (e.g. having M_(n) ˜200-400), hydroxy terminatedpoly(dimethylsiloxane) (e.g. having M_(n) ˜500-10,000) and dihydroxyterminated poly(dimethylsiloxane-co-diphenylsiloxane), or may be apolyol which is commercially available under trade marks such asTerathane® 650 Polyether glycol, Terathane® 1000 Polyether glycol,Terathane® 2000 Polyether glycol, Desmophen® 1600U and Desmophen® 1900U.The polyol component is preferably present in the reaction mixture in anamount of from 1 to 50 wt %, based on the total weight of the reactants.

In a preferred embodiment, the polymer binder precursor may additionallycomprise a cross-linking agent. The cross-linking agent is amulti-functional cross-linking compound, and is preferably selected fromcompounds having a functionality of more than two. For example, thecross-linking agent may be selected from compounds having more than twofunctional groups such as hydroxy or isocyanate groups. Particularlypreferred cross-linking agents include monomers, oligomers orprepolymers having more than two isocyanate groups, and in particulartri- and tetra-functional compounds such as polymethylenepolyphenylisocyanates. Representative compounds are commerciallyavailable from Dow Chemical under the trade mark PAPI (e.g. PAPI 27 andPAPI 2940).

Another group of preferred cross-linking agents consists of compoundscontaining more than two hydroxy groups, and in particular tri- andtetra-functional compounds, such as 1,2,3-propantriol andpolycaprolactone triol (e.g. having M_(w) ˜300), and branchedpolyalcohol compounds available from Aldrich under the trade markDesmophen (e.g. Desmophen 550U, Desmophen 1910U, Desmophen 1145 andDesmophen 1150).

These cross-linking agents may be employed in the reaction scheme in anyamount sufficient to provide compositions having the desired materialproperties, but are preferably present in an amount of from 0.1 to 10 wt%.

Many catalysts can be used to catalyse the reaction of the isocyanateand polyol components. Preferred catalysts are those based on metalssuch as tin and zinc, and in particular tin (II) octate and zinc (II)octate catalysts. Dibutyltin dilaurate is also preferred. The amount ofcatalyst may be determined by those skilled in the art, but ispreferably from about 0.01 to 5 wt %, based on the total weight of thereactants.

In a typical cross-linked type polyurethane synthesis, the procedurenormally consists of four components: (1) a monomeric or oligomericdiisocyanate, (2) a monomeric or oligomeric diol, (3) a cross-linkingagent, and (4) a catalyst. An exemplary synthetic scheme is shown inScheme 3 below.

The R groups in this scheme can be any type of repeating units, forexample a straight-chain or branched alkyl group. They may be based on anumber of molecules such as polyethylene glycol, polydimethylsiloxane,polytetrahydrofuran (e.g. Terathane® Polyether glycol) or polyester. Thepreferred groups are those based on low molecular Terathane® Polyetherglycol, polyethylene glycol and polydimethylsiloxane.

Polyhydrosilylation:

The addition polymerisation reaction used to prepare the polymer binderof the invention may alternatively be a polyhydrosilylation reaction.Hydrosilylation is a well-known methodology in organic synthesis whichtakes place under mild conditions. In a hydrosilylation reaction Si—Hand terminal carbon-carbon double bonds react to produce variousalkylsilane compounds.

Prepolymers or oligomers which may be used in such a reaction includematerials selected from the group consisting of 1,5-hexandiene,1,7-octandiene, 1,4-divinylbenzene, 1,3-divinylbenzene,1,3-divinyl-1,1,3,3-tetramethyldisiloxane,1,3-divinyl-1,1,3,3-tetraphenyldisiloxane,1,3-divinyl-1,3-dimethyl-1,3-diphenyldisiloxane,1,3-divinyl-1,1-dimethyl-3,3-diphenyldisiloxane, poly(dimethylsiloxane)divinyl terminated, poly(dimethylsiloxane-co-diphenylsiloxane) divinylterminated and poly(dimethylsiloxane-co-methylphenylsiloxane) divinylterminated (these compounds providing the C═C bond necessary for thehydrosilylation reaction).

Other preferred prepolymers or oligomers include compounds selected fromthe group consisting of 1,4-bis(dimethylsilyl)benzene,1,3-Bis(dimethylsilyl)benzene, 1,1,3,3-tetramethyldisiloxane,1,1,3,3-tetraphenyldisiloxane, 1,3-dimethyl-1,3-diphenyldisiloxane,1,1-dimethyl-3,3-diphenyldisiloxane, poly(dimethylsiloxane) dihydrideterminated, dihydride terminatedpoly(dimethylsiloxane-co-diphenylsiloxane) and dihydride terminatedpoly(dimethylsiloxane-co-methylphenylsiloxane), (these componentsproviding the Si—H bond necessary for the hydrosilylation reaction).

In one example, the hydrosilylation reaction can involve the reaction ofa hydrosilane and a vinylsilane which can react together to form apolycarbosilane polymer binder. In a preferred polymer synthesis,hydrosilylation occurs between α,ω-divinyl compounds andα,ω-bis(hydrosilyl) compounds in the presence of suitable cross-linkingagents and catalysts, to afford thermally stable polycarbosilanes.

In a general hydrosilylation methodology, vinylsilanes and hydrosilanesare reacted together, usually in the presence of a transition metalcatalyst (see scheme 4). The reaction involves the addition of Si—Hbonds (e.g. from the hydrosilane component) to C═C double bonds (e.g.from the vinylsilane component).

R and R′ can be selected from a number of substituents known to those inthe art, although alkyl groups and phenyl groups are particularlypreferred. In particular, methyl, ethyl, propyl, butyl or phenyl groupsmay be used.

Hydrosilylation methodology can thus be used for the preparation of thepolycarbosiloxanes or polycarbosilanes, which contain Si—C bonds in thepolymer backbone structures. Such polycarbosiloxanes or polycarbosilanescan be obtained by reaction of a di-, tri-, tetra-, orpoly-methylhydrosilyl-containing organopolysiloxane, and a di-, tri-,tetra-, or poly-vinyl-containing organopolysiloxane. The resultingpolycarbosiloxane or polycarbosilane polymeric binders are preferablythermoset polymers or rubbers, and are electrical insulators (that is,they have an electrical conductivity of generally less than about 10⁻⁹(Ω·cm)⁻¹).

The methylhydrosilyl-containing organopolysiloxane component preferablycomprises a compound of formula (a), (b) or (c):

In the formulae (a), each R group is independently selected from asubstituted or unsubstituted monovalent hydrocarbon group. Each R grouppreferably contains from 1 to 8 carbon atoms, with exemplary groupsincluding alkyl groups such as methyl, ethyl, propyl, butyl, hexyl andoctyl, aryl groups such as phenyl, tolyl and naphthyl, and aralkylgroups such as benzyl and phenylethyl. The preferred R groups are methyland phenyl groups. The letter x is an integer which can range between 0and 1000, and is preferably between 0 and 200.

Reactive monomers like 1,1,3,3-tetramethyldisiloxane,1,1,3,3-tetraphenyldisiloxane, 1,3-dimethyl-1,3-diphenyldisiloxane,1,1-dimethyl-3,3-diphenyldisiloxane, poly(di-methylsiloxane) dihydrideterminated, poly(dimethylsiloxane-co-diphenylsiloxane) dihydrideterminated and poly(dimethylsiloxane-co-methylphenylsiloxane) dihydrideterminated are particularly useful in this present invention.

In formula (b), R is as defined above in relation to formula (a), n isan integer of at least 2, m is an integer inclusive of 0, and the sum ofn plus m is between 3 to 8. Reactive molecules like2,4,6,8-tetramethylcyclotetrasiloxane, pentamethylcyclopentasiloxane and2,4,6,8,10,12-hexamethylcyclohexasiloxane are possible compounds offormula (b).

In formula (c), R is as defined above, and each R′ group isindependently a hydrogen atom, or a methyl or trimethylsilyl group. Theproportion of x (i.e. the wt % of the methylhydrosiloxane monomer unit)in the molecule ranges from 10 to 95 wt %, preferably from 10 to 50 wt%.

The vinyl-containing organopolysiloxane preferably has a formulaaccording to either of the formulae (d), (e) and (f) below:

Formula (d) represents a vinyl-terminated siloxane component whereineach R group is independently selected from a substituted orunsubstituted monovalent hydrocarbon group. The R group preferablycontains from 1 to 8 carbon atoms, with preferred examples includingalkyl groups such as methyl, ethyl, propyl, butyl, hexyl and octyl, andaryl groups such as phenyl, tolyl and naphthyl. The letter x is 0 or aninteger ranging from 1 to 1000, and is preferably between 0 and 200, andthe proportion of the vinyl group (in wt % based on the total weight ofthe vinyl-containing organopolysiloxane component) ranges from 0.1 to 30wt %.

Formula (e) represents a vinyl-containing cycloorganosiloxane component,wherein n is an integer of at least 2, m is an 0 or an integer between 1and 6, and the sum of n plus m is between 4 and 8.

In formula (f), each R group is as defined above in relation to formula(d), and each R′ group is independently selected from allyl, vinyl,methyl, and trimethylsilyl groups. The value of y is chosen such thatthe proportion of the vinyl-containing organosiloxane monomer unit (i.e.the wt % of this component relative to the total weight of the compound)ranges from 1 to 50 wt %. The preferred range is from 1 to 10 wt %.

The linear silicon-containing polymers (e.g. siloxanes or silanes)described above possess a highly robust chemical structure withexceptional thermal resistance properties. The thermal resistance can befurther improved by incorporating some cross-linking agent in thepolymer binder precursor. Potential cross-linking agents include tri-,tetra- or poly-methylhydrosilyl (or -vinyl) containing organosiloxanecross-linking components, which are preferably employed in the presenceof a platinum catalyst system. These provide additional heat resistanceperformance, resulting in compositions with excellent thermal stabilityperformance. A typical reaction scheme is shown below in Scheme 5:

In this scheme P can be any suitable linking group. For example, it maybe chosen such that the methylhydrosilyl component is the same as theproduct formed in scheme 4 above. P may be repeating unit based on anysuitable group known to those in the art, for example it may consist ofa polyethylene glycol-, polydimethylsiloxane- orpolyterahydrofuran-based group.

The cross-linking component (alternatively called a multifunctionalmonomer) is a molecule which contains three or more reactive functionalunits. For example, these functional units may be carbon-carbon singlebonds or C═C double bonds (e.g. a vinyl group), or hydrogen-silicon orcarbon-silicon bonds (e.g. a methylhydrosilyl group). Each of thesereactive functional units (e.g. C═C, or H—Si) can independently act asan active site and subsequently participate in the growth of the polymerchain, while the other reactive functional units can facilitate theformation of the branched polymer which grows and eventually connects toanother chain in order to create a cross-linked polymer structure. Thus,the multifunctional components mentioned above are capable of producinghighly cross-linked structures. Preferred cross-linking components arethose having three or more reactive functional units which are eithervinyl groups or methyhydrosilyl groups.

Thus, in a preferred embodiment, the present invention provides across-linked polycarbosiloxane polymer binder composition comprising thereaction product of:

-   -   (1) a di-, tri-, tetra-, or poly-methylhydrosilyl-containing        organopolysiloxane;    -   (2) a di-, tri-, tetra-, or poly-vinyl-containing        organopolysiloxane;    -   (3) a tri-, tetra-, or poly-vinyl-containing, or a tri-, tetra-,        or poly-methylhydrosilyl-containing organosiloxane cross-linking        component.

The cross-linking component is preferably present in liquid form, and ispreferably in an amount of from 0.1 to 10 wt %, based on the totalweight of the total composition.

Preferably the polymer binder composition further comprises anorganometallic catalyst. Suitable catalysts include platinum catalysts,which may be selected from chloroplatinic acid or platinum vinylsiloxanecomplexes. Platinum complexes are particularly advantageous in that theylower the reaction temperatures required for thermal curing. Thecatalyst is preferably present in an amount of from 0.01 to 5 wt %,based on the total weight of the polymer.

Other Components

In addition to the polymer binder component, overvoltage protectionmaterials of the present invention comprise a conductive material, whichis preferably present in the form of conductive particles dispersedthroughout the polymer matrix. The overvoltage protection materials mayalso include other components, for example semi-conductive materialsand/or non-conductive materials.

The conductive polymeric material composite may be formed as amultifunctional polymeric composite matrix system, having three or morecomponents. One particularly preferred system can be represented by theformula:A_(x)B_(y)C_(z)D_(n)where A is a polymeric binder (e.g. an insulating polymer matrixcomprising silicon rubber or other type of cross-linked polymer), B is aconductive material (e.g. aluminium, nickel or iron particles), C is anon-conductive material (e.g. inorganic particles that control thespacing between the conductive particles), and D is a semi-conductivematerial (e.g. inorganic particles which modulate the electricalproperties of the system and increase the heat dispersion effect). Thenature of components A, B, C and D, and the relevant proportions ofthese components (represented by x, y, z and n) can be altered in orderto achieve the desired overvoltage protection material.

The preferred values of x, y, z and n are those which result in apolymer matrix where A is present in an amount of from 10 to 60 wt %, Bis present in an amount of from 10 to 70 wt %, C is present in an amountof from 1 to 40 wt %, and D is present in an amount of from 1 to 40 wt%.

(i) Conductive Materials

The ability of the overvoltage protection materials to protect devicesfrom transient high voltage surges is dependent upon a number offactors, including the electrical properties of the polymeric binder,and the size of the gap between the electrodes of the resultingovervoltage protection devices. The nature of the conductive materialalso influences the properties of the overvoltage protection materials.For example, the electrical conductivity of the conductive material, thesize of the particles of this material, and the loading of conductivematerial within the polymer binder (and hence the interparticleseparation of the conductive material) all influence the overvoltageprotection capabilities.

The conductive materials used in the present invention have bulkconductivities greater than 1000 (Ω·cm)⁻¹, and are preferably present inthe form of conductive particles. These conductive particles generallyhave an average particle size (APS) of generally less than 20 microns,such as 0.1 to 20 microns, and more preferably less than 10 microns. Aparticularly preferred average particle size is between 1 and 10microns. Conductive particles having average particle sizes between 1and 30 microns are easily obtainable from commercial suppliers.

A wide range of suitable conductive particles is available. Examplesinclude metallic particles such as aluminium, copper, gold, iron,silver, beryllium, bismuth, cobalt, magnesium, molybdenum, palladium,tantalum, tungsten and tin particles; particles of metal alloys such asstainless steel, bronze and brass; carbide powders such as carbides oftitanium, boron, tungsten and tantalum; carbon-based powders such ascarbon black and graphite; metal nitrides and metal borides. Recently,intrinsically conducting polymers such as polyaniline and polypyrrolehave become available, and these would also be of use in the invention.Preferred conductive particles include nickel, aluminium, copper, carbonblack, graphite, gold, iron, stainless steel, silver, tin and metalalloys, with nickel and aluminium being most preferred.

The conductive material may be present in any amount necessary toachieve the desired overvoltage protection capabilities. Generally itwill be present in an amount of from 10 to 70 wt %, based on the totalweight of the composition, more preferably from 20 to 60 wt %, mostpreferably 30 to 55%.

(ii) Semi-Conductive Materials

The semi-conductive materials used in the present invention generallyhave bulk conductivities less than 100 (Ω·cm)⁻¹. They are employed tomodulate the electrical property of the overvoltage protectionmaterials, and to increase the heat dispersion effect.

The materials are preferably particulate inorganic materials, and theaverage particle size of the semi-conductive materials is usually lessthan 10 microns, for example from 0.1 to 10 microns, and more preferablythe average particle size is less than 2 microns, such as 0.1 to 2microns.

Suitable semi-conductive materials include aluminium nitride, bariumtitanate, boron nitride, silicon nitride, titanium dioxide, siliconcarbide and zinc oxide, with silicon carbide being preferred. Whenpresent in the composition, the semi-conductive materials are preferablypresent in an amount of from 1 to 40 wt %, based on the total weight ofthe composition, more preferably from 10 to 30 wt %.

(iii) Non-Conductive Materials

The non-conductive materials used in the present invention generallyhave bulk conductivities less than 10⁻⁶ (Ω·cm)⁻¹. They are employedprimarily to control the spacing between particles of the conductivematerial.

The non-conductive materials are present in the form of particles, withthe average particle size generally being less than 5 microns, forexample 0.005 to 10 microns, and more preferably being within the range0.01 to 1 micron. These materials are preferable organic materials, andare suitably selected from non-conducting materials (or inorganicfillers) including coated- or uncoated-aluminium oxide, barium oxide,silica, barium carbonate, calcium carbonate, magnesium carbonate,calcium sulphate and magnesium sulphate. Particularly preferred aresilica, aluminium oxide and calcium carbonate.

When present in the composition, the non-conductive materials preferablycomprise from 1 to 40 wt %, based on the total weight of thecomposition, more preferably from 5 to 20 wt %.

Processes for Preparing Overvoltage Protection Materials:

The overvoltage protection material of the invention may be producedaccording to the general processes described earlier. Due to the choiceof the polymer binder precursor, substantially no solvent is generatedduring the curing step to form the polymer binder, and this has a numberof advantages. For example, by reducing the amount of solvent generated,there is less solvent to evaporate during curing, hence less energy isrequired for the curing step. Also, reducing the amount of solventgenerated results in a material having fewer voids in its structure. Theovervoltage protection materials of the present invention preferablyhave substantially no voids.

The overvoltage protection materials may be prepared by a standardmixing technique of the components using conventional apparatus. A highspeed (e.g. >2000 rpm) continuous stirring tank reactor is particularlysuitable.

According to a preferred embodiment of the invention, there is provideda solvent free process of forming an overvoltage protection material,wherein a highly cross-linked polymer matrix is prepared having aconductive material dispersed throughout. The polymer matrix isgenerated from a polymer binder precursor which reacts via an additionpolymerisation reaction, as described earlier. Other components, such assemi-conductive and/or non-conductive materials may also be includedwithin resulting overvoltage protection material. This is a ‘one-pot’synthetic technique, in which all the materials are mixed or blendedtogether in a mixing vessel, and this reaction mixture is then heated inorder to form the polymer binder (also called the polymer matrix) havinga conductive material dispersed therein. The one-pot methodology has theadvantage that it reduces the number of processing steps, and hencereduces costs. Suitable synthetic routes are shown in the Examplesbelow.

Device Fabrication

The overvoltage protection devices of the invention may be preparedaccording to a process comprising depositing a metallic layer on asubstrate, and etching the metallic layer in order to provide metallicelectrodes separated by gaps; preparing a solvent-free mixturecomprising at least one polymer binder precursor and a conductivematerial, and applying this mixture between and in contact with adjacentelectrodes; and heating the device to cause reaction of the polymerbinder precursor and generate an overvoltage protection materialcomprising a polymer binder having conductive material dispersedtherein; wherein the polymer binder precursor is chosen such thatsubstantially no solvent is generated during the reaction. The polymerbinder precursor and conductive material are as discussed above.

The polymer binder precursor preferably reacts via an additionalpolymerisation reaction, and the polymer binder formed during thisprocess is substantially free of voids. The particular components (e.g.polymer binder precursor and conductive material) used to make thedevice can be optimised according to the application for which theresulting device will be used. However, it is preferred to choose thecomponents such that the overvoltage protection device has a triggervoltage of less than 300V. The devices preferably have a very fastswitching time, for example less than 1 nanosecond.

The substrate and metallic layer will be chosen depending on thespecific application of the device. For example, the device may be usedin a semiconductor device, and the substrate may then be selected frommaterials known in the art to be suitable for this application. Themetallic layer is preferably between about 1 to 20 microns thick, morepreferably from about 5 to about 15 microns thick. The metallicmaterials suitable for use as the electrodes are known to those skilledin the art, and include nickel, silver and copper, with nickel andcopper being preferred. The deposition step can be any conventionalmethod for depositing a thin layer of metal onto a substrate. Possiblemethods include sputtering, nickel-electrodless plating, electroplating,thermal evaporation and metal-epoxy lamination.

A suitable fabrication procedure is as follows. A thin metallic layer iscoated onto a substrate surface by any suitable method, such assputtering, thermal evaporation or electroplating. Alternatively, acommercially available copper laminated board, where a layer of copperis already present on a substrate, may be used. Exemplary substratesinclude, but are not limited to, laminated epoxy fibre glass (e.g. FR4),glass and ceramics.

The device pattern is formed on the surface of the device by a number ofsuitable procedures, for example by lithographic patterning or wetetching. The resulting gaps between the electrodes are generally of theorder of microns, for example between 10 and 1000 microns. A preferredelectrode separation is between 10 and 200 microns, more preferablybetween 30 and 150 microns.

An exemplary device, prior to addition of the overvoltage protectionmaterial, is shown in FIG. 2, which depicts two electrodes (12, 13) on asubstrate (14). The electrodes are separated by micro-gap (11).

Once the basic device structure has been fabricated, the solvent-freemixture, comprising the polymer binder precursor and the conductivematerial, can be applied onto the above mentioned device by a number ofdispensing methods such as hand brushing, screen printing and otherscasting procedure. Hand brushing is used in the following examples,although screen printing is preferred for large-scale production. Afterthe material has been applied to the device, the polymer binder must becured in order to form the final overvoltage protection device. Suitablecuring conditions will be known to those skilled in the art, buttemperatures of between 40 and 150° C., more preferably between 25 and100° C., have been found to be particularly suitable for making devicesaccording to the invention.

The finished device (10), after addition of the overvoltage protectionmaterial (15), is shown in FIG. 1 (cross-sectional view) and FIG. 3(plan view).

The overvoltage protection device may be fabricated as part of a largercircuit. For example, the invention provides a circuit comprisingelectronic components, at least a first and second conducting region, anamount of an overvoltage protection material being disposed between andin contact with said conducting regions.

The following Examples are intended to illustrate the present invention,and are not intended to limit the invention in any way.

EXAMPLES

A number of formulations were been prepared by a mixing technique whichis described in the examples below. After curing of the overvoltageprotection material, devices comprising these materials were tested. Inparticular, the devices were tested to determine: (1) electricalresistance; (2) trigger voltage (using the Human Body Model (HBM), astandard methodology for generating a transient voltage, in which thevoltage is generated by an ionised Charge Plate); (3) leakage current(recorded by Current-Voltage (I-V) method) measured by scanning avoltage between 0 to 24 V (d.c.) through the overvoltage protectiondevice and then taking the current reading at 12V; and (4) capacitance(measured at 1 MHz frequency). The devices were tested using anelectrical circuit as shown in FIG. 4, in order to determine the triggervoltage. FIG. 4 depicts a pulse generator (16), an overvoltageprotection device according to the invention (17), a resistor (18), aground (19) and a high speed oscilloscope (20).

Example 1

A conductive polymeric composite material comprising polyurethane as apolymeric binder was prepared according to the following formulation:Poly(1,4-butanediol), isophorone diisocyanate terminated 24.5 wt % Terathane 650, polyester glycol   5 wt % Polycaprolactone triol, Mw ˜3000.5 wt % Dibutytin dilaurate 0.6 wt % Nickel powder, APS 2.2-3.0 micron 45 wt % Calcium carbonate powder, APS 40 nm   5 wt % Silicon carbide(12S), APS 0.7 micron  20 wt %

In a typical preparation procedure using a polyurethane polymericbinder, poly(1,4-butanediol), isophorone diisocyanate terminated,Terathane 650 polyester glycol, polycaprolactone triol, nickel powder,calcium carbonate powder and silicon carbide powder were mixed in around bottomed flask and stirred with mechanical stirrer for 10-20minutes until the materials were well mixed using a high speed(e.g. >2000 rpm) continuous stirring tank reactor, and a viscousmaterial was obtained. Then, a catalytic amount of dibutyltin dilaurate(e.g. 0.6 wt % based on the polymer content) was added into the wellmixed viscous mixture, and the composition was stirred at roomtemperature for a further 10-15 minutes.

The resulting viscous material was cast onto the gap between electrodeson a device as shown in FIG. 1 by a normal hand casting method. Thedevice was then placed in a conventional hot-air oven at 80° C. to curethe polymer. After about 30 minutes curing the overvoltage protectiondevice was removed and its electrical properties measured as describedabove. The results are shown below in Table 1.

Example 2

A conductive polymeric composite material was prepared according to thefollowing formulation: Poly(dimethylsiloxane-co-methylhydrosiloxane)38.25 wt %  Polydimethylsiloxane, vinyldimethyl terminated 4.5 wt %1,3,5,7-Tetramethylcyclotetrasiloxane 2.25 wt %  Platinum vinylsiloxanecatalyst (Pt 0.1 wt %) 1.3 wt % Aluminium powder, APS 6.0 micron  40 wt% Calcium carbonate powder, APS 40 nm 7.5 wt % Alumina, AKP20, APS0.3-0.6 micron 7.5 wt %

In a typical preparation procedure,poly(dimethylsiloxane-co-methylhydrosiloxane), polydimethylsiloxane(vinyldimethyl terminated), 1,3,5,7-tetramethylcyclotetra-siloxane,aluminium powder, calcium carbonate powder and alumina powder were mixedin a round bottom flask and stirred with mechanical stirrer for 10-20minutes at 60° C. until the materials were well mixed. Then, a catalyticamount of platinum vinylsiloxane containing 0.1 wt % platinum (e.g. ˜1wt % based on polymer content) was added into the well mixed viscousmixture. The mixture was stirred at room temperature for a further 10-15minutes.

The resulting viscous material was cast onto the gap between electrodeson a device as shown in FIG. 1 by a normal hand casting method. Thedevice was then placed in a conventional hot-air oven at 80° C. to curethe polymer. After about one hour of curing the overvoltage protectiondevice was removed and its electrical properties measured as describedabove. The results are shown below in Table 1, from which it can be seenthat the overvoltage protection device had essentially the samecharacteristics as the overvoltage protection device in Example 1.

Example 3

A conductive polymeric composite material was prepared according to thefollowing formulation: Poly(dimethylsiloxane-co-methylhydrosiloxane)38.25 wt %   Polydimethylsiloxane, vinyldimethyl terminate 4.5 wt % 1,3,5,7-Tetramethylcyclotetrasiloxane 2.25 wt %   Platinum vinylsiloxanecatalyst (Pt 0.1 wt %) 1.3 wt %  Nickel powder, APS 2.2-3.0 micron 33 wt% Calcium carbonate powder, APS 40 nm 11 wt % Alumina, AKP20, APS0.3-0.6 micron 11 wt %

Exactly the same procedure was used as described above in Example 2, butusing the formulation above, where nickel powder with APS 2.2-3.0 micronwas used instead of aluminium powder. After curing, the resultingovervoltage protection device had essentially the same characteristicsas the protection device in Example 1 (see Table 1).

Example 4

A conductive polymeric composite material was prepared according to thefollowing formulation: Poly(dimethylsiloxane-co-methylhydrosiloxane) 30wt % Polydimethylsiloxane, vinyldimethyl terminate 10 wt % Platinumvinylsiloxane catalyst (Pt 0.1 wt %) 1.3 wt %  Aluminium powder, APS 6.0micron 50 wt % Calcium carbonate powder, APS 40 nm  5 wt % Siliconcarbide (7S), APS  5 wt %

Again, the procedure described in Example 2 was followed, but in thisexample, the formulation contained an additional component of siliconcarbide powder. The electrical properties of the resulting device areshown in Table 1. The overvoltage protection material composite was alittle softer than the composites of Examples 2 and 3 because anadditional crossing linking agent was omitted from the formulation

Example 5

A conductive polymeric composite material was prepared according to thefollowing formulation: Poly(dimethylsiloxane-co-methylhydrosiloxane)27.8 wt %  Polydimethylsiloxane, vinyldimethyl terminate 9.2 wt %Platinum vinylsiloxane catalyst (Pt 0.1 wt %) 1.3 wt % Aluminium powder,APS 6.0 micron  50 wt % Calcium carbonate powder, APS 40 nm 6.5 wt %Silicon carbide (7S), APS 6.5 wt %

The procedure of Example 2 was again followed, but with a differentformulation. After curing, the electrical properties of the resultingovervoltage protection device were measured, and the results are shownbelow in Table 1.

Example 6

A conductive polymeric composite material was prepared according to thefollowing formulation: Poly(dimethylsiloxane-co-methylhydrosiloxane)26.25 wt %  Polydimethylsiloxane, vinyldimethyl terminate 8.75 wt % Platinum vinylsiloxane catalyst (Pt 0.1 wt %) 1.3 wt % Aluminium powder,APS 6.0 micron  50 wt % Calcium carbonate powder, APS 40 nm 7.5 wt %Silicon carbide (7S), APS 7.5 wt %

The procedure of Example 2 was again followed, but with a differentformulation. After curing, the electrical properties of the resultingovervoltage protection device were measured, and the results are shownbelow in Table 1.

Example 7

A conductive polymeric composite material was prepared according to thefollowing formulation: Poly(dimethylsiloxane-co-methylhydrosiloxane)26.25 wt %   Polydimethylsiloxane, vinyldimethyl terminate 8.75 wt %  Platinum vinylsiloxane catalyst (Pt 0.1 wt %) 1.3 wt %  Aluminiumpowder, APS 6.0 micron 50 wt % Calcium carbonate powder, APS 40 nm 10 wt% Silicon carbide (7S), APS  5 wt %

The procedure of Example 2 was again followed, but with a differentformulation. After curing, the electrical properties of the resultingovervoltage protection device were measured, and the results are shownbelow in Table 1.

Example 8

A conductive polymeric composite material was prepared 26.25 wt %Polydimethylsiloxane, vinyldimethyl terminate  8.75 wt % Platinumvinylsiloxane catalyst (Pt 0.1 wt %)  1.3 wt % Aluminium powder, APS 6.0micron   50 wt % Calcium carbonate powder, APS 40 nm 11.28 wt % Siliconcarbide (7S), APS  3.72 wt %

The procedure of Example 2 was again followed, but with a differentformulation. After curing, the electrical properties of the resultingovervoltage protection device were measured, and the results are shownbelow in Table 1. TABLE 1 Resistance Leakage Trigger off state currentCapacitance Gap between Example Voltage (GΩ) Response (nA) (pF)electrodes No. (V) (@ 6 V) Time (ns) (@12 VDU) (@1 MHz) (μm) 1 214 6 <1<1 0.40 35 2 192 142 <1 <1 0.20 140 3 259 160 <1 <1 0.14 100 4 240 3 <1˜0.001 0.55 60 5 213 1 <1 ˜0.014 0.50 35 6 200 1 <1 ˜0.025 0.43 35 7 186121 <1 ˜0.033 0.44 35 8 207 138 <1 ˜0.010 0.47 35

The preceding examples describe the starting mixtures, preparationmethods and process conditions used, and the characteristics of theresulting conductive polymeric composite materials, as normalfabrication procedures according to the present invention. Theseexamples are illustrative of but a few of the many possible permutationsof chemicals and process parameters that could be used to prepareconductive polymeric composite materials of the invention.

The foregoing is offered primarily for the purposes of illustration. Itwill be readily apparent to those skilled in the art that numerousvariations, modifications and substitutions may be made in thematerials, procedural steps and conditions described herein withoutdeparting from the spirit and scope of the invention.

1-36. (canceled)
 37. An overvoltage protection device comprising anovervoltage protection material preparable according to a processcomprising: (i) preparing a mixture comprising at least one polymerbinder precursor and a conductive material, wherein the polymer binderprecursor is chosen such that substantially no solvent is generatedduring the reaction; and (ii) heating the mixture to cause reaction ofthe polymer binder precursor and generate a polymer matrix havingconductive material dispersed therein. 38-48. (canceled)
 49. Anovervoltage protection device comprising an overvoltage protectionmaterial comprising a polymer binder having a conductive materialdispersed therein, said overvoltage protection material being obtainableby an addition polymerisation reaction of a mixture comprising a polymerbinder precursor and a conductive material, wherein the polymer binderprecursor is chosen such that substantially no solvent is generatedduring the reaction.
 50. An overvoltage protection device according toclaim 49 wherein the overvoltage protection material is substantiallyfree of voids.
 51. An overvoltage protection device comprising anovervoltage protection material comprising a polymer binder having aconductive material dispersed therein, said material being substantiallyfree of voids, wherein the overvoltage protection material is obtainableby an addition polymerisation reaction of a mixture comprising a polymerbinder precursor and a conductive material.
 52. An overvoltageprotection device according to claim 51 wherein the overvoltageprotection is obtainable by an addition polymerisation reaction of amixture comprising a polymer binder precursor and a conductive material.53. A circuit comprising electronic components, at least a first andsecond conducting region, an amount of an overvoltage protectionmaterial being disposed between and in contact with said conductingregions, wherein said overvoltage protection material preparableaccording to a process comprising: (i) preparing a mixture comprising atleast one polymer binder precursor and a conductive material; and (ii)heating the mixture to cause reaction of the polymer binder precursorand generate a polymer matrix having conductive material dispersedtherein wherein the polymer binder precursor is chosen such thatsubstantially no solvent is generated during the reaction.
 54. A circuitcomprising electronic components, at least a first and second conductingmaterial, an overvoltage protection device being disposed between and incontact with said conducting regions, wherein said overvoltageprotection device comprises an overvoltage protection materialcomprising a polymer binder having a conductive material dispersedtherein, said overvoltage protection material being obtainable by anaddition polymerisation reaction of a mixture comprising a polymerbinder precursor and a conductive material, wherein the polymer binderprecursor is chosen such that substantially no solvent is generatedduring the reaction.